Hydraulic Lifter (Hydraulic Tappet)

Hydraulic lifters, tappets or lash adjusters contain a small plunger and a check valve. They are used in most modern petrol engines and many diesels. They rely on clean oil of the right viscosity and enough oil pressure to work properly. Some engines use hydraulic lash adjusters at the pivot of a roller rocker instead of tappets directly under the cam. The principle is the same.

Oil fills a chamber inside the lifter through a check valve. The trapped oil makes the lifter rigid while the valve opens, and a small, controlled leak lets it extend or shorten slightly to take up any gap.

Ticking from the top of the engine, especially at cold start or with low oil, can mean a lifter is not pumping up. Correct oil level and regular oil changes help prevent this. A brief tick for a few seconds after a cold start can be normal while oil pressure builds. A tick that stays when warm needs checking.